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In the ovaries are cell clusters called graff ovarian follicles, where are the germ cells, which originate the gametes, and the follicular cells, responsible for maintaining germ cells and producing female sex hormones.
In women, only one ovarian follicle matures at each menstrual cycle, a period between two consecutive menstruations and lasting on average 28 days. This means that with each cycle, only one gamete becomes mature and is released into a woman's reproductive system.
The ovaries alternate in maturation of their folliclesThat is, with each menstrual cycle, the release of an egg, or ovulation, happens in one of the two ovaries.
Oogenesis is divided into three stages:
Multiplication or proliferation phase: It is a phase of consecutive mitoses, when the germ cells increase in quantity and give rise to ovaries. In human female fetuses, the proliferative phase ends around the end of the first trimester of pregnancy. So by the time a girl is born, she already has about 400,000 Graff follicles in her ovaries. It is a limited amount, unlike men, which produce spermatogonia for most of their lives.
Growth stage: As soon as they are formed, the ovogonia begin the first division of meiosis, interrupted in prophase I. They then undergo remarkable growth, with increased cytoplasm and large accumulation of nutritive substances. This cytoplasmic nutrient deposit is called calf, and is responsible for the nutrition of the embryo during its development.
At the end of the growth phase, the ovaries become primary oocytes (first order oocytes or oocytes I). In women, this phase lasts until puberty, when the girl begins her sexual maturity.
Maturation Phase: Of the 400,000 primary oocytes, only 350 or 400 will complete their transformation into mature gametes, one each menstrual cycle. The maturation phase begins when the girl reaches sexual maturity, around 11 to 15 years old.
When the primary oocyte completes the first meiosis division, interrupted in prophase I, it produces two cells. One of them does not receive cytoplasm and then disintegrates, most often without initiating the second division of meiosis. It is the first polar corpuscle (or globule).
The other large, rich calf cell is the secondary oocyte (second order oocyte or oocyte II). When suffering, the second division of meiosis gives rise to the second polar corpuscle, which also dies in a short time, and the egg, the female gamete, a large, calf-filled cell.
In female gametogenesis, the meiotic division is uneven because it does not evenly divide the cytoplasm between daughter cells. This allows the egg formed to be quite rich in nutritive substances.
In most mammalian females, the second division of meiosis only happens if the gamete is fertilized. Curiously, the real gamete of these females is oocyte IIbecause it fuses with the sperm.